Bicomponent fibers with improved curvature

ABSTRACT

Provided are bicomponent fibers with improved curvature. The bicomponent fiber comprises a first region and a second region. The first region comprises a first polyethylene composition and second region comprises a second polyethylene composition, wherein the first polyethylene composition has a crystallization temperature (Tc) greater than a crystallization temperature (Tc) of the second polyethylene composition. The bicomponent fiber can be used to form a nonwoven.

TECHNICAL FIELD

Embodiments of the present disclosure generally relate to bicomponent fibers with improved curvature that comprise polyethylene, and nonwovens comprising the fibers.

INTRODUCTION

Bicomponent fibers are fibers made of two different polymer compositions that are extruded from the same spinneret with both compositions contained within the same filament or fiber. When the fiber leaves the spinneret, it consists of non-mixed components that are fused at the interface. The two polymer compositions can differ in their chemical and/or physical properties. Bicomponent fibers can be formed by conventional spinning techniques known in the art and can be used for forming a nonwoven. Nonwoven fabrics have numerous applications, such as filters, disposable materials in medical applications, and diaperstock. To assist in reducing nonwoven weight or obtaining other advantageous nonwoven properties, such as loft, bicomponent fibers having curvature can be used. However, problems exist with obtaining bicomponent fibers with improved curvature and with maintaining or improving other advantageous properties, such as spinnability, softness, recyclability, and extensibility, while improving curvature.

SUMMARY

Embodiments of the present disclosure provide bicomponent fibers that can be used to form nonwovens and that provide in aspects unique and surprisingly high curvature, while also maintaining or improving other properties such as spinnability, tactile softness, recyclability, and extensibility. Bicomponent fibers according to embodiments of the present disclosure each include a first region and a second region comprising a first polyethylene composition and a second polyethylene composition, respectively, that contribute to a fiber with improved curvature and advantageous spinnability, softness, recyclability, and extensibility. Specifically, bicomponent fibers according to embodiments of the present disclosure comprise a first polyethylene composition and a second polyethylene composition that can improve spinnability, softness, recyclability, and extensibility, and can interface to improve the inherent curvature of the fibers (e.g., fiber curvature that is not the result of mechanical crimping or a post-extrusion process, such as attenuation with heated air or application of tension).

Disclosed herein is a bicomponent fiber. In one embodiment, the bicomponent fiber comprises a fiber centroid; a first region having a first centroid and a second region having a second centroid; the first region comprising a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of less than 3.0; the second region comprising a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid; and wherein the first polyethylene composition has a crystallization temperature (Tc) at least 2° C. greater than a crystallization temperature (Tc) of the second polyethylene composition.

In a different embodiment, the bicomponent fiber comprises a fiber centroid; a first region having a first centroid and a second region having a second centroid; the first region comprising a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of greater than 3.0; the second region comprising a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid; wherein the first polyethylene composition has a crystallization temperature (Tc) at least 3.5° C. greater than a crystallization temperature (Tc) of the second polyethylene composition.

Also disclosed herein are nonwovens formed from the bicomponent fiber disclosed herein. For example, a spunbond nonwoven can be formed from the bicomponent fiber disclosed herein. In one embodiment, the spunbond nonwoven comprises a bicomponent fiber comprising a fiber centroid; a first region having a first centroid and a second region having a second centroid; the first region comprising a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of less than 3.0; the second region comprising a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid; and wherein the first polyethylene composition has a crystallization temperature (Tc) at least 2° C. greater than a crystallization temperature (Tc) of the second polyethylene composition. In a different embodiment, the spunbond nonwoven comprises a bicomponent fiber comprising a fiber centroid; a first region having a first centroid and a second region having a second centroid; the first region comprising a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of greater than 3.0; the second region comprising a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid; wherein the first polyethylene composition has a crystallization temperature (Tc) at least 3.5° C. greater than a crystallization temperature (Tc) of the second polyethylene composition.

Additional features and advantages of the embodiments will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing and the following description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph (SEM) cross-section image of a bicomponent fiber having an eccentric core-sheath configuration and centroid off-set.

FIG. 2 is an illustration of a single reactor stream feed data flow used to produce a polyethylene composition disclosed herein.

FIG. 3 is an illustration of a dual reactor stream feed data flow used to produce a polyethylene composition disclosed herein.

DETAILED DESCRIPTION

Aspects of the disclosed bicomponent fibers are described in more detail below. The bicomponent fibers having increased curvature can be used to form nonwovens, and such nonwovens can have a wide variety of applications, including, for example, wipes, face masks, tissues, bandages, and other medical and hygiene products. It is noted however, that this is merely an illustrative implementation of the embodiments disclosed herein. The embodiments are applicable to other technologies that are susceptible to similar problems as those discussed above.

As used herein, the terms “comprising,” “including,” “having,” and their derivatives, are not intended to exclude the presence of any additional component, step or procedure, whether or not the same is specifically disclosed. In order to avoid any doubt, all compositions claimed through use of the term “comprising” may include any additional additive, adjuvant, or compound, whether polymeric or otherwise, unless stated to the contrary. In contrast, the term, “consisting essentially of” excludes from the scope of any succeeding recitation any other component, step or procedure, excepting those that are not essential to operability. The term “consisting of” excludes any component, step or procedure not specifically delineated or listed.

As used herein, the term “interpolymer” refers to polymers prepared by the polymerization of at least two different types of monomers. The term interpolymer thus includes copolymers (employed to refer to polymers prepared from two different types of monomers), and polymers prepared from more than two different types of monomers.

As used herein, the term “polymer” means a polymeric compound prepared by polymerizing monomers, whether of the same or a different type. The generic term polymer thus embraces the term homopolymer (employed to refer to polymers prepared from only one type of monomer, with the understanding that trace amounts of impurities can be incorporated into the polymer structure), and the term interpolymer. Trace amounts of impurities (for example, catalyst residues) may be incorporated into and/or within the polymer. A polymer may be a single polymer or polymer blend.

As used herein, the term “polyethylene composition” refers to a polymer comprising greater than 50% by weight of units which are derived from ethylene monomer, and optionally, one or more comonomers. A polyethylene composition includes polyethylene homopolymers, copolymers, or interpolymers. Common forms of polyethylene compositions known in the art include Low Density Polyethylene (LDPE); Linear Low Density Polyethylene (LLDPE); Ultra Low Density Polyethylene (ULDPE); Very Low Density Polyethylene (VLDPE); single-site catalyzed Linear Low Density Polyethylene, including both linear and substantially linear low density resins (m-LLDPE); Medium Density Polyethylene (MDPE); and High Density Polyethylene (HDPE).

As used herein, the terms “nonwoven,” “nonwoven web,” and “nonwoven fabric” are used herein interchangeably. “Nonwoven” refers to a web or fabric having a structure of individual fibers or threads which are randomly interlaid, but not in an identifiable manner as is the case for a knitted fabric.

As used herein, the term “meltblown” refers to the fabrication of a nonwoven fabric via a process which includes the following steps: (a) extruding molten thermoplastic strands from a spinneret; (b) simultaneously quenching and attenuating the polymer stream immediately below the spinneret using streams of high velocity heated air; (c) collecting the drawn strands into a web on a collecting surface. Meltblown nonwoven webs can be bonded by a variety of means including, but not limited to, autogeneous bonding (i.e., self bonding without further treatment), thermo-calendaring process, adhesive bonding process, hot air bonding process, needle punch process, hydroentangling process, and combinations thereof.

As used herein, the term “spunbond” refers to the fabrication of a nonwoven fabric including the following steps: (a) extruding molten thermoplastic strands from a plurality of fine capillaries called a spinneret; (b) quenching the strands of the thermoplastic strands comprising, for example, a polyethylene composition, with a flow of air which is generally cooled in order to hasten the solidification of the molten strands of the thermoplastic; (c) attenuating the filaments by advancing them through the quench zone with a draw tension that can be applied by either pneumatically entraining the filaments in an air stream or by winding them around mechanical draw rolls of the type commonly used in the textile fibers industry; (d) collecting the drawn strands into a web on a foraminous surface (e.g., moving screen or porous belt); and (e) bonding the web of loose strands into the nonwoven fabric. Bonding can be achieved by a variety of means including, but not limited to, thermo-calendaring process, adhesive bonding process, hot air bonding process, needle punch process, hydroentangling process, and combinations thereof.

As used herein, the term “curvature” refers to the curve or crimp of an individual fiber that is result of its composition and not the result of any post-extrusion process that can impact the curve or crimp of the fiber (e.g., mechanical crimping or attenuation by heat). The amount of curvature of the bicomponent fibers disclosed herein can be measured in accordance with the test method described below.

Fibers

The fibers taught herein may be formed by any conventional spinning technique. For example, the first region and the second region of a bicomponent fiber can be formed into a fiber via melt spinning. In melt spinning, the first region comprising a first polyethylene composition and second region comprising a second polyethylene composition can be melted, coextruded, and forced through the fine orifices in a metallic plate, called a spinneret, into air or other gas, where they are cooled and solidified forming a bicomponent fiber. The solidified fiber may be drawn off via air jets, rotating rolls, or godets, and can be laid on a conveyer belt as a web for forming a nonwoven. A meltblown nonwoven comprising a bicomponent fiber according to embodiments of the present disclosure can be formed. In other embodiments, a spunbond nonwoven comprising a bicomponent fiber according to embodiments of the present disclosure can be formed.

The fibers disclosed herein have improved curvature and advantageous other properties, such as recyclability, tactile softness, and extensibility as a result of being comprised of polyethylene. The improved curvature of the fibers disclosed herein is not the result of mechanical crimping or a post-extrusion process, such as attenuation with heated air or application of tension. The fibers in aspects include all or a majority polyethylene compositions. Nonwovens comprising polyethylene compositions are known for their tactile softness, and materials comprising polyethylene compositions are candidates for compatibility with polyethylene recycling streams.

In embodiments, the bicomponent fiber has a curvature of at least 0.50 mm⁻¹. The curvature of the bicomponent fiber can be measured in accordance with the test method described below. All individual values and subranges of at least 0.50 mm⁻¹ are disclosed and included herein. For example, in some embodiments, the bicomponent fiber can have a curvature of at least 0.50, 0.60, 0.70, or 0.80 mm⁻¹, when measured according to the test method described below. In other embodiments the bicomponent fiber can have a curvature in the range of from 0.50 to 3.00, from 0.50 to 2.50, from 0.50 to 2.00, from 0.50 to 1.50, from 0.50 to 1.00, from 1.00 to 3.00, from 1.00 to 2.50, from 1.00 to 2.00, from 1.00 to 1.50, from 1.50 to 3.00, from 1.50 to 2.50, from 1.50 to 2.00, from 2.00 to 3.00, or from 2.00 to 2.50 mm⁻¹, when measured according to the test method described below.

In embodiments, the bicomponent fiber comprises a first region and a second region, wherein the weight ratio of the first region to the second region is 90:10 to 10:90. All individual values and subranges of a ratio of from 90:10 to 10:90 are disclosed and included herein. For example, in embodiments, the weight ratio of the first region to the second region can be from 80:20 to 20:80, from 70:30 to 30:70, from 60:40 to 40:60, or from 55:45 to 45:55.

Although the fibers taught herein are bicomponent fibers, a person of ordinary skill in the art will appreciate that because the two regions of the fibers both contain polyethylene compositions, it may not be readily discernable from the fiber itself that it includes two different regions. A person of ordinary skill in the art will appreciate that Raman microscopy and multivariate calibration, as described in the below test method section, can be used to measure, in situ, the percent (%) crystallinity of individual polyethylene regions of the bicomponent fibers. The difference between the Raman measured % crystallinity of the two regions of the bicomponent fibers according to embodiments of the present disclosure corresponds to the improved curvature of the fibers. In embodiments, the first polyethylene composition of the first region of the bicomponent fiber has a Raman measured % crystallinity at least 5.0% greater than a Raman measured % crystallinity of the second polyethylene composition of the second region of the bicomponent fiber, where Raman measured % crystallinity is measured in accordance with the test method described below. All individual values and subranges at least 5.0% greater than are disclosed and included herein; for example, the first polyethylene composition of the first region of the bicomponent fiber can have a Raman measured % crystallinity at least 5.0% greater than, at least 7.5% greater than, at least 10.0% greater than, or from 5.0% to 20.0% greater than, from 5.0% to 15.0% greater than, from 7.5% to 15.0% greater than, from 10.0% to 15.0% greater than, from 3.5% to 12.0% greater than, from 5.0% to 12.0% greater than, from 7.5% to 12.0% greater than, or from 10.0% to 12.0% greater than a Raman measured % crystallinity of the second polyethylene composition of the second region of the bicomponent fiber, where Raman measured % crystallinity is measured in accordance with the test method described below.

Centroids

In embodiments, the bicomponent fiber comprises a fiber centroid and a first region having a first centroid and a second region having a second centroid, wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid.

As used herein, the term “centroid” refers to the arithmetic mean of all the points of a region of a cross-section of a bicomponent fiber. For example, the bicomponent fiber according to embodiments of the present disclosure has a fiber centroid, which can be designated as C_(f), and a region of the bicomponent fiber (e.g., the first or second region) has an independent centroid, which can be designated as C_(rx), where x is a designation of the region (e.g., the first region can be designated as C_(r1) and the second region can be designated as C_(r2)), and where “r” is the average distance from C_(f) to the outer surface of the bicomponent fiber and is calculated as √{square root over (A/π)}, where A is the area of the bicomponent fiber cross-section. FIG. 1 illustrates a bicomponent fiber and its centroid as well as the centroid of the second region of the bicomponent fiber. The distance from a region centroid to the fiber centroid can be defined as “P_(rx)”, and the centroid offset of the first centroid or second centroid to the fiber centroid can be defined as “P_(rx)/r.”

In embodiments, at least one of the first centroid and the second centroid is not the same as the fiber centroid. Where the first centroid or the second centroid are different than the fiber centroid, the bicomponent fiber can have different configurations, such as eccentric core-sheath, side-by-side, or segmented pie, but cannot have a concentric configuration (e.g., a core-sheath concentric configuration) where the fiber centroid, first centroid, and the second centroid are the same. In embodiments, the first centroid of the first region and the second centroid of the second region are arranged such that the first region and the second region are in a side-by-side configuration. In other embodiments, the first centroid of the first region and the second centroid of the second region are arranged such that the first region and the second region are in a segmented pie configuration. In further embodiments, the first centroid of the first region and the second centroid of the second region are arranged such that the first region and the second region are in an eccentric core-sheath configuration, where the first region is the sheath of the bicomponent fiber and the second region is the core region of the bicomponent fiber and the sheath region surrounds the core region.

In embodiments, the first centroid or the second centroid is offset from the fiber centroid by at least 0.1, or at least 0.2, or at least 0.4, and is less than 1 or less than 0.9, where offset is measured in accordance with the test method described below.

First and Second Regions—Metallocene or Single Site Catalyst Embodiments

In certain embodiments, the bicomponent fiber comprises a first region and a second region; the first region comprises a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of less than 3.0; the second region comprises a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein the first polyethylene composition has a crystallization temperature (Tc) at least 2° C. greater than a crystallization temperature (Tc) of the second polyethylene composition. In such embodiments, the first polyethylene composition can be formed in the presence of a metallocene or single site catalyst.

Further, in such embodiments, the first polyethylene composition has a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of less than 3.0. All individual values and subranges of a molecular weight distribution (M_(w(GPC))/M_(n(GPC))) of less than 3.0 are disclosed and included herein; for example, in embodiments, the first polyethylene composition has a molecular weight distribution (M_(w(GPC))/M_(n(GPC))) of less than 3.0, less than 2.8, less than 2.6, less than 2.4, or less than 2.2, or from a range of from 1.8 to 3.0, 1.8 to 2.6, 1.8 to 2.4, 1.8 to 2.2, 2.0 to 3.0, 2.0 to 2.6, 2.0 to 2.4, 2.0 to 2.2, 2.2 to 2.6, or 2.2 to 2.4, where molecular weight distribution can be expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))) and can be measured in accordance with the test method described below.

Further, in such embodiments, the first polyethylene composition has a crystallization temperature (Tc) at least 2° C. greater than a crystallization temperature (Tc) of the second polyethylene composition. All individual values and subranges of a crystallization temperature (Tc) of the first polyethylene composition at least 2° C. greater than a crystallization temperature (Tc) of the second polyethylene composition are disclosed and included herein; for example, the first polyethylene composition can have a crystallization temperature (Tc) of at least 2° C. greater than, at least 4° C. greater than, at least 6° C. greater than, at least 8° C. greater than, at least 10° C. greater than, at least 12° C. greater than, at least 14° C. greater than, at least 16° C. greater than, or at least 18° C. greater than a crystallization temperature (Tc) of the second polyethylene composition, or the difference between a crystallization temperature (Tc) of the first polyethylene composition minus a crystallization temperature (Tc) of the second polyethylene composition can be in the range of from 2° C. to 30° C., 2° C. to 25° C., 2° C. to 20° C., 2° C. to 15° C., 2° C. to 10° C., 2° C. to 5° C., 5° C. to 30° C., 5° C. to 25° C., 5° C. to 20° C., 5° C. to 15° C., 5° C. to 10° C., 10° C. to 30° C., 10° C. to 25° C., 10° C. to 20° C., 10° C. to 15° C., 15° C. to 30° C., 15° C. to 25° C., or 15° C. to 20° C., where crystallization temperature (Tc) can be measured according to Differential Scanning calorimetry (DSC) as described below.

Further, in such embodiments, the first polyethylene composition can have a melting temperature (Tm) at least 2° C. greater than a melting temperature (Tm) of the second polyethylene composition. All individual values and subranges of a melting temperature (Tm) of the first polyethylene composition at least 2° C. greater than a melting temperature (Tm) of the second polyethylene composition are disclosed and included herein; for example, the first polyethylene composition can have melting temperature (Tm) of at least 2° C. greater than, at least 4° C. greater than, at least 6° C. greater than, at least 8° C. greater than, at least 10° C. greater than, at least 14° C. greater than, at least 18° C. greater than, at least 22° C. greater than, or at least 26° C. greater than, or at least 30° C. greater than a melting temperature (Tm) of the second polyethylene composition, or the difference between a melting temperature (Tm) of the first polyethylene composition minus a melting temperature (Tm) of the second polyethylene composition can be in the range of from 2° C. to 50° C., 2° C. to 45° C., 2° C. to 40° C., 2° C. to 35° C., 2° C. to 30° C., 2° C. to 25° C., 2° C. to 20° C., 2° C. to 15° C., 2° C. to 10° C., 2° C. to 5° C., 5° C. to 50° C., 5° C. to 45° C., 5° C. to 40° C., 5° C. to 35° C., 5° C. to 30° C., 5° C. to 25° C., 5° C. to 20° C., 5° C. to 15° C., 5° C. to 10° C., 10° C. to 50° C., 10° C. to 40° C., 10° C. to 30° C., 10° C. to 20° C., 20° C. to 50° C., 20° C. to 40° C., 20° C. to 30° C., 25° C. to 50° C., 25° C. to 40° C., 25° C. to 35° C., 30° C. to 50° C., 30° C. to 40° C., 30° C. to 35° C., or 30° C. to 32° C., where melting temperature can be measured according to DSC as described below.

Further, in such embodiments, the melting temperature (Tm) of the first polyethylene composition can be less than 130° C. All individual values and subranges of less than 130° C. are disclosed and included herein; for example, the melting temperature (Tm) of the first polyethylene composition can be less than 130° C., less than 129.8° C., less than 129.6° C., less than 129.4° C., less than 129.2° C., less than 129° C., or less than 128.9° C., where melting temperature (Tm) can be measured according to DSC as described below. In embodiments, the melting temperature (Tm) of the second polyethylene composition can be less than 127° C. All individual values and subranges of less than 127° C. are disclosed and included herein; for example, the melting temperature (Tm) of the second polyethylene composition can be less than 127° C., less than 126.5° C., less than 125° C., less than 120° C., less than 115° C., less than 110° C., less than 105° C., less than 100° C., less than 99° C., less than 98.5° C., or less than 98° C., where melting temperature (Tm) can be measured according to DSC as described below.

In embodiments, the difference between the melting temperature (Tm) of the first polyethylene composition minus the melting temperature (Tm) of the second polyethylene composition can be at least 1.5° C. All individual values and subranges of at least 1.5° C. are included and disclosed herein; for example, the difference between the melting temperature (Tm) of the first polyethylene composition minus the melting temperature (Tm) of the second polyethylene composition can be at least 1.5° C., at least 2.0° C., at least 2.5° C., at least 3° C., at least 5° C., at least 10° C., at least 15° C., at least 20° C., at least 25° C., or at least 30° C., or can be in the range of from 1.5° C. to 40° C., 2.0° C. to 40° C., 2.5° C. to 40° C., 1.5° C. to 30° C., 2.0° C. to 30° C., 2.5° C. to 30° C., 1.5° C. to 20° C., 2.0° C. to 20° C., 2.5° C. to 20° C., 1.5° C. to 10° C., 2.0° C. to 10° C., 2.5° C. to 10° C., 1.5° C. to 5° C., 2.0° C. to 5° C., 2.5° C. to 5° C., 10° C. to 40° C., 10° C. to 35° C., 10° C. to 30° C., 10° C. to 20° C., 20° C. to 40° C., 20° C. to 35° C., 20° C. to 30° C., 25° C. to 40° C., 25° C. to 35° C., 28° C. to 32° C., or 29° C. to 31° C., where melting temperature (Tm) can be measured according to DSC as described below.

First and Second Regions—Ziegler Natta Catalyst Embodiments

In other embodiments, the bicomponent fiber comprises a first region and a second region; the first region comprises a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of greater than 3.0; the second region comprises a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein the first polyethylene composition has a crystallization temperature (Tc) at least 3.5° C. greater than a crystallization temperature (Tc) of the second polyethylene composition. In such embodiments, the first polyethylene composition can be formed in the presence of a Ziegler-Natta catalyst. In such embodiments, the first polyethylene composition has a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of greater than 3.0. All individual values and subranges of a molecular weight distribution (M_(w(GPC))/M_(n(GPC))) of greater than 3.0 are disclosed and included herein; for example, in embodiments, the first polyethylene composition has a molecular weight distribution (M_(w(GPC))/M_(n(GPC))) of greater than 3.0, greater than 3.02, greater than 3.04, greater than 3.06, greater than 3.08, greater than 3.10, greater than 3.12, or greater than 3.14, or from a range of from 3.0 to 5.0, 3.0 to 4.5, 3.0 to 4.0, 3.0 to 3.5, 3.0 to 3.2, 3.1 to 5.0, 3.1 to 4.5, 3.1 to 4.0, 3.1 to 3.5, or 3.1 to 3.2, where molecular weight distribution can be expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))). In such embodiments, the first polyethylene composition can be formed in the presence of a Ziegler Natta catalyst.

Further, in such embodiments, the first polyethylene composition has a crystallization temperature (Tc) at least 3.5° C. greater than a crystallization temperature (Tc) of the second polyethylene composition. All individual values and subranges of a crystallization temperature (Tc) of the first polyethylene composition at least 3.5° C. greater than a crystallization temperature (Tc) of the second polyethylene composition are disclosed and included herein; for example, the first polyethylene composition can have a crystallization temperature (Tc) of at least 3.5° C. greater than, at least 4° C. greater than, at least 4.5° C. greater than, at least 5° C. greater than, at least 5.5° C. greater than, at least 6° C. greater than, at least 6.2° C. greater than, or at least 6.4° C. greater than a crystallization temperature (Tc) of the second polyethylene composition, or the difference between a crystallization temperature (Tc) of the first polyethylene composition minus a crystallization temperature (Tc) of the second polyethylene composition can be in the range of from 3.5° C. to 15° C., 3.5° C. to 10° C., 3.5° C. to 7.5° C., 3.5° C. to 6° C., 5° C. to 15° C., 5° C. to 10° C., 5° C. to 7.5° C., 5° C. to 6° C., 6° C. to 15° C., 6° C. to 10° C., 6° C. to 8° C., or 6° C. to 7° C., where (Tc) can be measured according to DSC as described below.

Further, in such embodiments, the first polyethylene composition can have a melting temperature (Tm) at least 5° C. greater than a melting temperature (Tm) of the second polyethylene composition. All individual values and subranges of a melting temperature (Tm) of the first polyethylene composition at least 5° C. greater than a melting temperature (Tm) of the second polyethylene composition are disclosed and included herein; for example, the first polyethylene composition can have melting temperature (Tm) of at least 5° C. greater than, at least 5.2° C. greater than, at least 5.4° C. greater than, at least 5.6° C. greater than, at least 5.8° C. greater than, at least 6.0° C. greater than, at least 6.2° C. greater than, and at least 6.4° C. greater than, at least 6.6° C. greater than, at least 6.8° C. greater than, or at least 6.9° C. greater than a melting temperature (Tm) of the second polyethylene composition, or the difference between a melting temperature (Tm) of the first polyethylene composition minus a melting temperature (Tm) of the second polyethylene composition can be in the range of from 5° C. to 10° C., 5° C. to 7.5° C., 5° C. to 7° C., 5° C. to 6.5° C., 5° C. to 6° C., 5.5° C. to 10° C., 5.5° C. to 7.5° C., 5.5° C. to 7° C., 5.5° C. to 6° C., 6° C. to 10° C., 6° C. to 7.5° C., 6° C. to 7° C., 6.5° C. to 10° C., 6.5° C. to 7.5° C., or 6.5° C. to 7° C., where melting temperature (Tm) can be measured according to DSC as described below.

First and Second Regions—Generally

In embodiments described herein, the second polyethylene composition has a density less than a density less than a density of the first polyethylene composition, where density can be measured according to ASTM D792. In some embodiments, the density of the first polyethylene composition is at least 0.015 g/cm³ greater than the density of the second polyethylene composition. All individual values and subranges at least 0.015 g/cm³ greater are included and disclosed herein; for example, in some embodiments, the density of the first polyethylene composition is at least 0.015 g/cm³, at least 0.030 g/cm³, or at least 0.040 g/cm³ greater than the density of the second polyethylene composition, or the difference between the density of the first polyethylene composition minus the density of the second polyethylene composition is in the range of from 0.015 g/cm³ to 0.100 g/cm³, 0.015 g/cm³ to 0.080 g/cm³, 0.015 g/cm³ to 0.060 g/cm³, 0.015 g/cm³ to 0.040 g/cm³, 0.015 g/cm³ to 0.020 g/cm³, 0.020 g/cm³ to 0.100 g/cm³, 0.020 g/cm³ to 0.080 g/cm³, 0.020 g/cm³ to 0.060 g/cm³, 0.020 g/cm³ to 0.040 g/cm³, 0.020 g/cm³ to 0.030 g/cm³, 0.030 g/cm³ to 0.100 g/cm³, 0.030 g/cm³ to 0.080 g/cm³, 0.030 g/cm³ to 0.060 g/cm³, 0.030 g/cm³ to 0.050 g/cm³, 0.030 g/cm³ to 0.040 g/cm³, or 0.040 g/cm³ to 0.050 g/cm³, where density can be measured according to ASTM D792.

In embodiments described herein, the first polyethylene composition can have a density of at least 0.925 g/cm³, where density can be measured according to ASTM D792. All individual values and subranges of a density in the range of at least 0.925 g/cm³ are disclosed and included herein. For example, in some embodiments, the first polyethylene composition can have a density of at least 0.935, at least 0.940, at least 0.945, at least 0.950, at least 0.955, at least 0.960, or at least 0.965 g/cm³, where density can be measured according to ASTM D792, or the first polyethylene composition can have a density in the range of from 0.925 to 0.980, from 0.930 to 0.980, from 0.940 to 0.980, from 0.950 to 0.980, from 0.930 to 0.980, from 0.930 to 0.970, from 0.930 to 0.960, from 0.930 to 0.950, from 0.940 to 0.980, from 0.940 to 0.970, from 0.940 to 0.960, from 0.940 to 0.950, from 0.945 to 0.980, from 0.945 to 0.970, from 0.945 to 0.960, from 0.945 to 0.955, from 0.950 to 0.980, from 0.950 to 0.970, from 0.950 to 0.960, from 0.960 to 0.980, or from 0.960 to 0.980 g/cm³, where density can be measured according to ASTM D792.

In embodiments described above and herein, the first region of the bicomponent fiber comprises at least 75 wt. % of the first polyethylene composition. All individual values and subranges of at least 75 wt. % are included and disclosed herein; for example, the first region can comprise at least 75 wt. %, at least 80 wt. %, at least 85 wt. %, at least 90 wt. %, at least 95 wt. %, or from 75 wt. % to 100 wt. %, from 75 wt. % to 90 wt. %, from 75 wt. % to 80 wt. %, from 80 wt. % to 100 wt. %, or from 90 wt. % to 100 wt. %, of the first polyethylene composition, where weight percent is based on the total weight of the first region.

In embodiments described above and herein, the second region of the bicomponent fiber comprises at least 75 wt. % of the second polyethylene composition. All individual values and subranges of at least 75 wt. % are included and disclosed herein; for example, the second region can comprise at least 75 wt. %, at least 80 wt. %, at least 85 wt. %, at least 90 wt. %, at least 95 wt. %, or from 75 wt. % to 100 wt. %, from 75 wt. % to 90 wt. %, from 75 wt. % to 80 wt. %, from 80 wt. % to 100 wt. %, or from 90 wt. % to 100 wt. %, of the second polyethylene composition, where weight percent is based on the total weight of the second region.

In embodiments described above and herein, the first region and/or the second region can comprise additional components, such as, one or more other polymers and/or one or more additives. Other polymers can include a polyester, another polyethylene composition, a propylene-based polymer (e.g. polypropylene homopolymer, propylene-ethylene copolymer, or propylene/alpha-olefin interpolymer), or a propylene-based plastomer or elastomer. The amount of the other polymer may be up to 25 wt. % based on the total weight of the first region or second region including such other polymers. For example, in embodiments, the first region and/or second region can comprise up to 25 wt. % of a propylene-based plastomer or propylene-based elastomer (such as VERSIFY™ polymers available from The Dow Chemical Company and VISTAMAXX™ polymers available from ExxonMobil Chemical Co.), low modulus and/or low molecular weight polypropylene (such as L-MODU™ polymer from Idemitsu), random copolypropylene, or propylene-based olefin block copolymers (such as Intune). Potential additives include, but are not limited to, antistatic agents, color enhancers, dyes, lubricants, fillers, pigments, primary antioxidants, secondary antioxidants, processing aids, UV stabilizers, anti-blocks, slip agents, tackifiers, fire retardants, anti-microbial agents, odor reducer agents, anti-fungal agents, and combinations thereof. The first region and/or second region can contain from about 0.01 or 0.1 or 1 to about 25 or about 20 or about 15 or about 10 weight percent by the combined weight of such additives, based on the weight of the first region or second region including such additives.

Polymerization

Any conventional polymerization processes can be employed to produce the first or second polyethylene composition. Such conventional polymerization processes include, but are not limited to, solution polymerization process, using one or more conventional reactors e.g. loop reactors, isothermal reactors, stirred tank reactors, batch reactors in parallel, series, and/or any combinations thereof. Such conventional polymerization processes also include gas-phase, solution or slurry polymerization or any combination thereof, using any type of reactor or reactor configuration known in the art.

In embodiments, the solution phase polymerization process occurs in one or more well-stirred reactors such as one or more loop reactors at a temperature in the range of from 115 to 250° C.; for example, from 155 to 225° C., and at pressures in the range of from 300 to 1000 psi; for example, from 400 to 750 psi. In one embodiment in a dual reactor, the temperature in the first reactor temperature is in the range of from 115 to 190° C., for example, from 115 to 150° C., and the second reactor temperature is in the range of 150 to 200° C., for example, from 170 to 195° C. In another embodiment in a single reactor, the temperature in the reactor temperature is in the range of from 115 to 250° C., for example, from 155 to 225° C. The residence time in a solution phase polymerization process is typically in the range of from 2 to 30 minutes; for example, from 10 to 20 minutes. Ethylene, solvent, one or more catalyst systems, optionally one or more cocatalysts, optionally one or more impurity scavengers, and optionally one or more comonomers are fed continuously to one or more reactors. Exemplary solvents include, but are not limited to, isoparaffins. For example, such solvents are commercially available under the name ISOPAR E from ExxonMobil Chemical Co., Houston, Tex. The resultant mixture of the first or second polyethylene composition and solvent is then removed from the reactor and the first or second polyethylene composition is isolated. Solvent is typically recovered via a solvent recovery unit, i.e. heat exchangers and vapor liquid separator drum, and is then recycled back into the polymerization system.

In one embodiment, the first or second polyethylene composition may be produced via a solution polymerization process in a dual reactor system, for example a dual loop reactor system, wherein ethylene and optionally one or more a-olefins are polymerized in the presence of one or more catalyst systems. In another embodiment, the first or second polyethylene composition may be produced via a solution polymerization process in a single reactor system, for example a single loop reactor system, wherein ethylene and optionally one or more a-olefins are polymerized in the presence of one or more catalyst systems. As noted above, in certain embodiments, the first polyethylene composition is formed in the presence of a metallocene or single site catalyst system. In other embodiments, the first polyethylene composition is formed in the presence of a Ziegler-Natta catalyst system.

An example of a catalyst system suitable for producing the second polyethylene composition can be a catalyst system comprising a procatalyst component comprising a metal-ligand complex of formula (I):

In formula (I), M is a metal chosen from titanium, zirconium, or hafnium, the metal being in a formal oxidation state of +2, +3, or +4; n is 0, 1, or 2; when n is 1, X is a monodentate ligand or a bidentate ligand; when n is 2, each X is a monodentate ligand and is the same or different; the metal-ligand complex is overall charge-neutral; each Z is independently chosen from −O—, —S—, —N(R^(N))—, or —P(R^(P))—, wherein independently each R^(N) and R^(P) is (C1-C30)hydrocarbyl or (C1-C30)heterohydrocarbyl; L is (C₁-C₄₀)hydrocarbylene or (C₁-C₄₀)heterohydrocarbylene, wherein the (C₁-C₄₀)hydrocarbylene has a portion that comprises a 1-carbon atom to 10-carbon atom linker backbone linking the two Z groups in Formula (I) (to which L is bonded) or the (C₁-C₄₀)heterohydrocarbylene has a portion that comprises a 1-atom to 10-atom linker backbone linking the two Z groups in Formula (I), wherein each of the 1 to 10 atoms of the 1-atom to 10-atom linker backbone of the (C₁-C₄₀)heterohydrocarbylene independently is a carbon atom or heteroatom, wherein each heteroatom independently is O, S, S(O), S(O)₂, Si(R^(C))₂, Ge(R^(C))₂, P(R^(C)), or N(R^(C)), wherein independently each R^(C) is (C₁-C₃₀)hydrocarbyl or (C₁-C₃₀)heterohydrocarbyl; R¹ and R⁸ are independently selected from the group consisting of —H, (C₁-C₄₀)hydrocarbyl, (C₁-C₄₀)heterohydrocarbyl, —Si(R^(C))₃, —Ge(R^(C))₃, —P(R^(P))₂, —N(R^(N))₂, —OR^(C), —SR^(C), —NO₂, —CN, —CF₃, R^(C)S(O)—, R^(C)S(O)₂—, (R^(C))₂C═N—, R^(C)C(O)O—, R^(C)OC(O)—, R^(C)C(O)N(R^(N))—, (R^(N))₂NC(O)—, halogen, and radicals having formula (II), formula (III), or formula (IV):

In formulas (II), (III), and (IV), each of R³¹⁻³⁵, R⁴¹⁻⁴⁸, or R⁵¹⁻⁵⁹ is independently chosen from (C₁-C₄₀)hydrocarbyl, (C₁-C₄₀)heterohydrocarbyl, —Si(R^(C))₃, —Ge(R^(C))₃, —P(R^(P))₂, —N(R^(N))₂, —N═CHR^(C), —OR^(C), —SR^(C), —NO₂, —CN, —CF₃, R^(C)S(O)—, R^(C)S(O)₂—, (R^(C))₂C═N—, R^(C)C(O)O—, R^(C)OC(O)—, R^(C)C(O)N(R^(N))—, (R^(N))₂NC(O)—, halogen, or —H, provided at least one of R¹ or R⁸ is a radical having formula (II), formula (III), or formula (IV) where R^(C), R^(N), and R^(P) are as defined above.

In formula (I), each of R²⁻⁴, R⁵⁻⁷, and R⁹⁻¹⁶ is independently selected from (C₁-C₄₀)hydrocarbyl, (C₁-C₄₀)heterohydrocarbyl, —Si(R^(C))₃, —Ge(R^(C))₃, —P(R^(P))₂, —N(R^(N))₂, —N═CHR^(C), —OR^(C), —SR^(C), —NO₂, —CN, —CF₃, R^(C)S(O)—, R^(C)S(O)₂—, (R^(C))₂C═N—, R^(C)C(O)O—, R^(C)OC(O)—, R^(C)C(O)N(R^(N))—, (R^(C))₂NC(O)—, halogen, and —H where R^(C), R^(N), and R^(P) are as defined above.

The catalyst system comprising a metal-ligand complex of formula (I) may be rendered catalytically active by any technique known in the art for activating metal-based catalysts of olefin polymerization reactions. For example, a metal-ligand complex of formula (I) may be rendered catalytically active by contacting the complex to, or combining the complex with, an activating co-catalyst. Suitable activating co-catalysts for use herein include alkyl aluminums; polymeric or oligomeric alumoxanes (also known as aluminoxanes); neutral Lewis acids; and non-polymeric, non-coordinating, ion-forming compounds (including the use of such compounds under oxidizing conditions). A suitable activating technique is bulk electrolysis. Combinations of one or more of the foregoing activating co-catalysts and techniques are also contemplated. The term “alkyl aluminum” means a monoalkyl aluminum dihydride or monoalkylaluminum dihalide, a dialkyl aluminum hydride or dialkyl aluminum halide, or a trialkylaluminum. Examples of polymeric or oligomeric alumoxanes include methylalumoxane, triisobutylaluminum-modified methylalumoxane, and isobutylalumoxane.

Lewis acid activators (co-catalysts) include Group 13 metal compounds containing from 1 to 3 (C₁-C₂₀)hydrocarbyl substituents as described herein. Examples of Group 13 metal compounds are tri((C₁-C₂₀)hydrocarbyl)-substituted-aluminum or tri(C₁-C₂₀)hydrocarbyl)-boron compounds; tri(hydrocarbyl)-substituted-aluminum, tri(C₁-C₂₀)hydrocarbyl)-boron compounds; tri(C₁-C₁₀)alkyl) aluminum, tri(C₆-C₁₈)aryl)boron compounds; and halogenated (including perhalogenated) derivatives thereof. In further examples, Group 13 metal compounds are tris(fluoro-substituted phenyl)boranes, tris(pentafluorophenyl)borane. An activating co-catalyst can be a tris((C₁-C₂₀)hydrocarbyl borate (e.g. trityl tetrafluoroborate) or a tri((C₁-C₂₀)hydrocarbyl) ammonium tetra((C₁-C₂₀)hydrocarbyl)borane (e.g. bis(octadecyl)methylammonium tetrakis(pentafluorophenyl)borane). As used herein, the term “ammonium” means a nitrogen cation that is a ((C₁-C₂₀)hydrocarbyl)₄N⁺, a ((C₁-C₂₀)hydrocarbyl)₃N(H)⁺, a ((C₁-C₂₀)hydrocarbyl)₂N(H)₂ ⁺, (C₁-C₂₀)hydrocarbylN(H)₃ ⁺, or N(H)₄ ⁺, wherein each (C₁-C₂₀)hydrocarbyl, when two or more are present, may be the same or different.

Combinations of neutral Lewis acid activators (co-catalysts) include mixtures comprising a combination of a tri(C₁-C₄)alkyl)aluminum and a halogenated tri(C₆-C₁₈)aryl)boron compound, especially a tris(pentafluorophenyl)borane; or combinations of such neutral Lewis acid mixtures with a polymeric or oligomeric alumoxane, and combinations of a single neutral Lewis acid, especially tris(pentafluorophenyl)borane with a polymeric or oligomeric alumoxane. Ratios of numbers of moles of (metal-ligand complex):(tris(pentafluoro-phenylborane):(alumoxane) [e.g., (Group 4 metal-ligand complex):(tris(pentafluoro-phenylborane):(alumoxane)] are from 1:1:1 to 1:10:30, or from 1:1:1.5 to 1:5:10.

The catalyst system comprising the metal-ligand complex of formula (I) can be activated to form an active catalyst composition by combination with one or more co-catalysts, for example, a cation forming co-catalyst, a strong Lewis acid, or combinations thereof. Suitable activating co-catalysts include polymeric or oligomeric aluminoxanes, especially methyl aluminoxane, as well as inert, compatible, noncoordinating, ion forming compounds. Exemplary suitable co-catalysts include, but are not limited to: modified methyl aluminoxane (MMAO), bis(hydrogenated tallow alkyl)methyl, tetrakis(pentafluorophenyl)borate(1⁻) amine, and combinations thereof.

One or more of the foregoing activating co-catalysts can be used in combination with each other. One preferred combination is a mixture of a tri((C₁-C₄)hydrocarbyl)aluminum, tri((C₁-C₄)hydrocarbyl)borane, or an ammonium borate with an oligomeric or polymeric alumoxane compound. The ratio of total number of moles of one or more metal-ligand complexes of formula (I) to total number of moles of one or more of the activating co-catalysts is from 1:10,000 to 100:1. The ratio can be at least 1:5000, or, at least 1:1000; and can be no more than 10:1 or no more than 1:1. When an alumoxane alone is used as the activating co-catalyst, preferably the number of moles of the alumoxane that are employed can be at least 100 times the number of moles of the metal-ligand complex of formula (I). When tris(pentafluorophenyl)borane alone is used as the activating co-catalyst, the ratio of the number of moles of the tris(pentafluorophenyl)borane that are employed to the total number of moles of one or more metal-ligand complexes of formula (I) can be from 0.5:1 to 10:1, from 1:1 to 6:1, or from 1:1 to 5:1. The remaining activating co-catalysts are generally employed in approximately mole quantities equal to the total mole quantities of one or more metal-ligand complexes of formula (I).

Test Methods Density

Density is measured in accordance with ASTM D792, and expressed in grams/cm³ (g/cm³).

Melt Index (I2)

Melt Index (I2) is measured in accordance with ASTM D1238 at 190° Celsius (° C.) and 2.16 kg, and is expressed in grams eluted/10 minutes (g/10 min).

Curvature

The amount of curvature is measured via optical microscopy. The amount of curvature is calculated based on the inverse of the radius of the helix formed by the fiber. This is equal to the radius of the circle formed by projection of the helix formed by the fiber on a surface perpendicular to it. Average value of at least 5 measurements is reported. Measurements are reported in units of 1/millimeter (mm⁻¹).

Centroid Off-Set

Fibers were embedded in epoxy and polished under cryogenic conditions using a Leica UCT/FCS microtome operated at −140° C. for AFM analysis. Topography and phase images were captured at ambient temperature by using a Bruker Icon AFM system with a MikroMasch probe. The probe has a spring constant of 40 N/m and a resonant frequency in the vicinity of 170 kHz. An imaging frequency of 0.5-2 Hz is used with a set point ratio of approximately 0.8. The diameter of a fiber's cross section is measured using a single cord, and this measurement is divided in half to mark a mid-point as the fiber centroid (Cf). The core region of the bicomponent fiber is divided with two cords at 90° to visually create four quadrants of equal areas, and the intersection of the two cords defines the centroid of the core region (Cr2). The distance between the fiber centroid (Cf) and the centroid of the core region (Cr2) is measured, and then is divided by the radius of the fiber to calculate the fiber centroid offset (Pr2/r).

Conventional GPC (Mw/Mn)

Conventional GPC is obtained by high temperature gel permeation chromatography (GPC) equipment (PolymerChar, Spain). The IR5 detector (“measurement channel”) is used as a concentration detector. GPCOne software (PolymerChar, Spain) is used to calculate weight-average (Mw), and number-average (Mn) molecular weight of the polymer and to determine molecular weight distribution (Mw/Mn). The method uses three 10 micron PL gel mixed B columns (Agilent Technologies, column dimension 100×7.6 mm) or four 20 micron PL gel mixed A columns (Agilent Technologies, column dimension 100×7.6 mm) operating at a system temperature of 150° C. Samples are prepared at a 2 mg/mL concentration in 1,2,4-trichlorobenzene solvent containing 200 part per million of antioxidant butylated hydroxytoluene (BHT) for 3 hours at 160° C. with a gentle shaking by autosampler (PolymerChar, Spain). The flow rate is 1.0 mL/min, the injection size is 200 microliters. GPCOne software is used to calculate the plate count. The chromatographic system must have a minimum of 22,000 plates.

The GPC column set is calibrated by running at least 20 narrow molecular weight distribution polystyrene standards. The calibration uses a third order fit for the system with three10 micron PL gel mixed B columns or a fifth order fit for the system with four 20 micron PL gel mixed A columns. The molecular weight (MW) of the standards range from 580 g/mol to 8,400,000 g/mol, and the standards are contained in 6 “cocktail” mixtures. Each standard mixture has approximately a decade of separation between individual molecular weights. The standard mixtures are purchased from Agilent Technologies. The polystyrene standards are prepared at “0.025 g in 50 mL of solvent” for molecular weights equal to, or greater than, 1,000,000 g/mol, and at “0.05 g in 50 mL of solvent” for molecular weights less than 1,000,000 g/mol. The polystyrene standards are dissolved at 80° C., with gentle agitation, for 30 minutes. The narrow standards mixtures are run first, and in order of decreasing highest molecular weight component, to minimize degradation. The polystyrene standard peak molecular weights are converted to polyethylene molecular weights using Equation (1) (as described in Williams and Ward, J. Polym. Sci., Polym. Letters, 6, 621 (1968)):

MW_(PE) =A×(MW_(PS))^(B)  (Eq. 1)

where MW is the molecular weight of polyethylene (PE) or polystyrene (PS) as marked, and B is equal to 1.0. It is known to those of ordinary skill in the art that A may be in a range of about 0.38 to about 0.44 such that the A value yields 52,000 MWPE for Standard Reference Materials (SRM) 1475a. Use of this polyethylene calibration method to obtain molecular weight values, such as the molecular weight distribution (MWD or Mw/Mn), and related statistics, is defined here as the modified method of Williams and Ward. The number-average molecular weight, the weight-average molecular weight, and the z-average molecular weight are calculated from the following equations.

M _(n,cc) =Σw _(i)/Σ(w _(i) /M _(cc,i))  (Eq. 2)

M _(w,cc) =Σw _(i) M _(cc,i)  (Eq. 3)

M _(z,cc)=Σ(w _(i) M _(cc,i) ²)/Σ(w _(i) /M _(cc,i))  (Eq. 4)

where M_(n,cc), M_(w,cc), and M_(z,cc) (in g/mole) are the number-, weight-, and z-average molecular weight obtained from the conventional calibration, respectively. w_(i) is the weight fraction of the polyethylene molecules eluted at retention volume V_(i). M_(cc,i) is the molecular weight (in g/mole) of the polyethylene molecules eluted at retention volume V_(i) obtained using the conventional calibration (see Equation (1)).

The chromatographic peaks should be set to include area marking a significant visible departure from baseline when the chromatogram is viewed at 20 percent peak height. The baseline should not be integrated to less than 100 polyethylene-equivalent molecular weight and care must be used to account for anti-oxidant mismatch from the prepared sample and the chromatographic mobile phase.

Use of a decane flow rate marker is shown in the IR5 chromatogram. At no point should the baseline (response) Y-value difference between the start and the end of the baseline be greater than 3 percent of the integrated peak height of the chromatogram. In such a case, the chromatographic sample must be handled through proper matching of the sample and mobile phase antioxidant.

w (wt. fraction greater than 10⁵ g/mole) is calculated according the MWD curve (w_(i) versus log M_(cc,i)) obtained from GPCOne software according to Equation (5)

$\begin{matrix} {w = {\int\limits_{{\log M_{{cc},i}} = 5}^{{\log M}_{{cc},i} = 7}{w_{i}d\log M_{{cc},i}/{\int\limits_{{\log M_{{cc},i}} = 2}^{{\log M}_{{cc},i} = 7}{w_{i}d\log M_{{cc},i}}}}}} & \left( {{Eq}.5} \right) \end{matrix}$

Differential Scanning Calorimetry (DSC)

DSC is used to measure the melting temperature (Tm) and crystallization temperature (Tc) behavior of a polymer over a wide range of temperatures. For example, the TA Instruments Q1000 DSC, equipped with an RCS (refrigerated cooling system) and an autosampler is used to perform this analysis. During testing, a nitrogen purge gas flow of 50 ml/min is used. Each sample is melt pressed into a thin film at about 175° C.; the melted sample is then air-cooled to room temperature (approx. 25° C.). The film sample is formed by pressing a “0.1 to 0.2 gram” sample at 175° C. at 1,500 psi, and 30 seconds, to form a “0.1 to 0.2 mil thick” film. A 3-10 mg, 6 mm diameter specimen is extracted from the cooled polymer, weighed, placed in a light aluminum pan (ca 50 mg), and crimped shut. Analysis is then performed to determine its thermal properties.

The thermal behavior of the sample is determined by ramping the sample temperature up and down to create a heat flow versus temperature profile. First, the sample is rapidly heated to 180° C., and held isothermal for five minutes, in order to remove its thermal history. Next, the sample is cooled to −40° C., at a 10° C./minute cooling rate, and held isothermal at −40° C. for five minutes. The sample is then heated to 150° C. (this is the “second heat” ramp) at a 10° C./minute heating rate. The cooling and second heating curves are recorded. The cool curve is analyzed by setting baseline endpoints from the beginning of crystallization to −20° C. The heat curve is analyzed by setting baseline endpoints from −20° C. to the end of melt. The values determined are highest peak melting temperature (referred herein as the “melting temperature (Tm)”), highest peak crystallization temperature (referred herein as the “crystallization temperature (Tc)”), heat of fusion (H_(f)) (in Joules per gram), and the calculated % crystallinity for polyethylene samples using: % Crystallinity=((H_(f))/(292 J/g))×100. The heat of fusion (H_(f)) and the melting temperature (Tm) are reported from the second heat curve. The crystallization temperature (Tc) is determined from the cooling curve.

Raman Microscopy

Raman microscopy and multivariate calibration is used to measure the % crystallinity of individual polyethylene regions of the bicomponent fibers, in situ. Raman microscopy, a type of vibrational spectroscopic technique, is sensitive to vibrations of the polymer backbone and can provide information on both the amorphous and crystalline phases of a polymer and polyethylene compositions. Raman can use visible or near-infrared radiation and when coupled with an optical microscope provides a lateral spatial resolution of approximately 0.8 to 1.2 micrometers (depending on the excitation laser and microscope objective used).

A Partial Least Square (PLS) model is built to correlate Raman data with the annealed base resin density and percent (%) crystallinity calculated from the annealed polyethylene composition density. Annealed density is measured in accordance with ASTM D792. Percent (%) crystallinity is calculated from the measured annealed density using the following equation (Equation 6):

$\begin{matrix} {{{wt}\%{{Cryst}.}} = {\frac{\rho_{c}}{\rho}\left( \frac{\rho - \rho_{a}}{\rho_{c} - \rho_{a}} \right)}} & \left( {{Eq}.6} \right) \end{matrix}$

Where: ρ_(a)=0.855 g/cc (100% amorphous) and ρ_(c)=1.000 g/cc (100% crystalline) densities.

Depolarized Raman spectra are acquired using equivalent Thermo Scientific DXR2 micro-Raman instruments. Raman spectra are acquired using a 900 grooves/mm grating. Spectral range covered a Raman shift from 50 to 3500 cm⁻¹, with a data spacing of 0.964 cm⁻¹. Other data acquisition parameters are as follows. Acquisition time: 3-10 sec; Number of acquisitions: 3 to 6; dark current subtraction, cosmic ray filter and white light correction: turned ON. Calibration data were recorded with an Olympus M PlanN 20× (0.40 NA) objective using a 25 micrometer slit and an Olympus M PlanN 100× (0.90 NA) objective using a 50 micrometer pinhole.

Twenty eight polyethylene composition resins ranging in density from 0.859-0.964 g/cm³ are used for calibration and cross-validation of the PLS model. The PLS model is also validated using an independent set of density plaques and then used to measure the resin crystallinity of resins used for regions of bicomponent fibers. The PLS model is built with TQ Analyst™ software using the following parameters: Spectral Region: 1571 cm¹ to 971 cm⁻¹; Normalization: Integrated Area 1356-1227 cm⁻¹—same baseline points; Total number of samples: 28; #Calibration standards: 26; #Independent Cross validation samples: 2; #Independent Validation samples: 6; Data pre-processing for: Annealed Base resin density model and Calc. % Crystallinity model—Mean centering, 2^(nd) derivative, SG smoothing (15 point, 3^(rd) order polynomial); Number of Factors Used for Calibration of both models: Annealed Base resin density and calc. % crystallinity=4.

After validation of the PLS model, a longitudinal (parallel to the draw direction) cross section of each bicomponent fiber example is prepared. The cross section is oriented on the sample stage such that the draw direction of the fiber is oriented in the East-West direction on the sample stage. Depolarized Raman spectra are acquired in three different locations of each region of the bicomponent fiber example using a 100× (0.9NA) objective and 25 micrometer pinhole. The resulting Raman spectra from each region are averaged and the average spectrum is used to measure the region % crystallinity using the PLS model.

Examples Synthesis of Polyethylene Compositions

Developmental resins (“Resin 1”, “Resin 2”, “Resin 3”, and “Resin 4”) are prepared according to the following process and tables.

All raw materials (monomer and comonomer) and the process solvent (a narrow boiling range high-purity isoparaffinic solvent, Isopar-E) are purified with molecular sieves before introduction into the reaction environment. Hydrogen is supplied pressurized as a high purity grade and is not further purified. The reactor monomer feed stream is pressurized via a mechanical compressor to above reaction pressure. The solvent and comonomer (if present) feed is pressurized via a pump to above reaction pressure. The individual catalyst components are manually batch diluted with purified solvent and pressured to above reaction pressure. All reaction feed flows are measured with mass flow meters and independently controlled with computer automated valve control systems.

Reactor configuration is either single reactor operation or dual series reactor operation as specified in Table 2.

Either a single reactor system or a two reactor system in a series configuration is used. Each reactor is a continuous solution polymerization reactor consisting of a liquid full, non-adiabatic, isothermal, circulating, loop reactor which mimics a continuously stirred tank reactor (CSTR) with heat removal. Independent control of all fresh solvent, monomer, comonomer (if present), hydrogen, and catalyst component feeds is possible. The total fresh feed stream to each reactor (solvent, monomer, comonomer [if present], and hydrogen) is temperature controlled typically between 15-50° C. to maintain a single solution phase by passing the feed stream through a heat exchanger. The total fresh feed to each polymerization reactor is injected into the reactor at two locations with approximately equal reactor volumes between each injection location. The fresh feed is controlled with each injector receiving half of the total fresh feed mass flow. The catalyst components are injected into the polymerization reactor through injection nozzles to introduce the components into the center of the reactor flow. The primary catalyst component feed is computer controlled to maintain the reactor monomer conversion at the specified values. The cocatalyst component(s) is/are fed based on calculated specified molar ratios to the primary catalyst component Immediately following each reactor feed injection location, the feed streams are mixed with the circulating polymerization reactor contents with static mixing elements. The contents of each reactor are continuously circulated through heat exchangers responsible for removing much of the heat of reaction and with the temperature of the coolant side responsible for maintaining an isothermal reaction environment at the specified temperature. Circulation around each reactor loop is provided by a pump.

In dual series reactor configuration the effluent from the first polymerization reactor (containing solvent, monomer, comonomer [if present], hydrogen, catalyst components, and polymer) exits the first reactor loop and is added to the second reactor loop.

In all reactor configurations the final reactor effluent (second reactor effluent for dual series or the single reactor effluent) enters a zone where it is deactivated with the addition of and reaction with a suitable reagent (water). At this same reactor exit location other additives are added for polymer stabilization.

Following catalyst deactivation and additive addition, the reactor effluent enters a devolatization system where the polymer is removed from the non-polymer stream. The isolated polymer melt is pelletized and collected. The non-polymer stream passes through various pieces of equipment which separate most of the ethylene which is removed from the system. Most of the solvent and unreacted comonomer (if present) is recycled back to the reactor after passing through a purification system. A small amount of solvent and comonomer (if present) is purged from the process.

The reactor stream feed data flows that correspond to the values in Table 2 used to produce the developmental resins are graphically described in FIG. 2 and FIG. 3 . The data are presented such that the complexity of the solvent recycle system is accounted for and the reaction system can be treated more simply as a once through flow diagram.

TABLE 1 Catalyst Components Primary Catalyst component 1

Primary Catalyst A Ziegler-Natta type catalyst. The heterogeneous Ziegler-Natta type component 2 catalyst-premix was prepared substantially according to U.S. Pat. No. 4,612,300, by sequentially adding to a volume of ISOPAR E, a slurry of anhydrous magnesium chloride in ISOPAR E, a solution of EtAlCl₂ in heptane, and a solution of Ti(O—iPr)₄ in heptane, to yield a composition containing a magnesium concentration of 0.20 M and a ratio of Mg/Al/Ti of 40/12.5/3. An aliquot of this composition was further diluted with ISOPAR-E to yield a final concentration of 500 ppm Ti in the slurry. An aliquot of this composition can be further diluted with ISOPAR-E if required. The catalyst premix was contacted with a dilute solution of Et₃Al, in the molar Al to Ti ratio specified in Table 2, to give the active catalyst. Primary Catalyst component 3

Primary Catalyst component 4

Co-catalyst A bis(hydrogenated tallow alkyl)methylammonium tetrakis(pentafluoro- phenyl)borate(1−) amine Co-catalyst B Triethyl aluminum Co-catalyst C Aluminoxanes, iso-Bu Me, branched, cyclic and linear; modified methyl aluminoxane

TABLE 2 Production Conditions Resin 1 Resin 2 Resin 3 Resin 4 Reactor Configuration Type Single Single Dual Series Dual Series Comonomer type Type 1-hexene none 1-octene 1-octene First Reactor Feed Solvent/ g/g 3.47 2.99 3.13 3.09 Ethylene Mass Flow Ratio First Reactor Feed Comonomer/ g/g 0.009 0.000 0.331 0.370 Ethylene Mass Flow Ratio First Reactor Feed Hydrogen/ g/g 3.58E−04 3.36E−04 1.86E−04 3.22E−04 Ethylene Mass Flow Ratio First Reactor Temperature ° C. 185 200 140 150 First Reactor Pressure barg 38 38 50 50 First Reactor Ethylene % 93.5 94.4 96.6 89.7 Conversion First Reactor Catalyst Type Type Primary Primary Primary Primary (See also Table 1) catalyst catalyst catalyst catalyst component component component component 1 2 3 3 First Reactor Co-Catalyst 1 Type Co-catalyst None Co-catalyst Co-catalyst Type (See also Table 1) A A A First Reactor Co-Catalyst 2 Type Co-catalyst Co-catalyst Co-catalyst Co-catalyst Type (See also Table 1) B B C C First Reactor Co-Catalyst 1 to mol/mol 1.0 (B/Zr) 1.4 (B/Zr) 1.5 (B/Zr) Catalyst Molar Ratio (B to Catalyst Metal ratio) First Reactor Co-Catalyst 2 to mol/mol 4.1 (Al/Zr) 3.6 (Al/Ti) 32 (Al/Zr) 68 (Al/Zr) Catalyst Molar Ratio (Al to Catalyst Metal ratio) First Reactor Residence Time min 12.6 14.1 21.2 18.4 Percentage of Total Ethylene wt % 33.5% 37.2% Feed to First Reactor Second Reactor Feed Solvent/ g/g 2.49 2.43 Ethylene Mass Flow Ratio Second Reactor Feed g/g 0.096 0.150 Comonomer/Ethylene Mass Flow Ratio Second Reactor Feed Hydrogen/ g/g 4.04E−04 3.14E−04 Ethylene Mass Flow Ratio Second Reactor Temperature ° C. 195 195 Second Reactor Pressure barg 53 51 Second Reactor Ethylene % 91.7 91.4 Conversion Second Reactor Catalyst Type Type Primary Primary (See also Table 1) catalyst catalyst component component 4 4 Second Reactor Co-Catalyst 1 Type Co-catalyst Co-catalyst Type (See also Table 1) A A Second Reactor Co-Catalyst 2 Type Co-catalyst Co-catalyst Type (See also Table 1) C C Second Reactor Co-Catalyst 1 to mol/mol 2.1 (B/Zr) 1.2 (B/Zr) Catalyst Molar Ratio (B to Catalyst Metal ratio) Second Reactor Co-Catalyst 2 to mol/mol 3615 (Al/Zr) 2403 (Al/Zr) Catalyst Molar Ratio (Al to Catalyst Metal ratio) Second Reactor Residence Time min 7.8 7.6

The following materials are used in the examples.

Polymer 1 (Poly. 1) is Resin 1 described above.

Polymer 2 (Poly. 2) is ASPUN™ 6835, a polyethylene composition and linear low density polyethylene fiber resin commercially available from The Dow Chemical Company (Midland, Mich.).

Polymer 3 (Poly. 3) is Resin 2 described above.

Polymer 4 (Poly. 4) is Resin 3 described above.

Polymer 5 (Poly. 5) is Resin 4 described above.

Polymer 6 (Poly. 6) is ELITE™ 5860, a polyethylene composition and an enhanced polyethylene resin commercially available from The Dow Chemical Company (Midland, Mich.).

Poly. 1 to Poly. 6 have a density, melt index (I2), molecular weight distribution (Mw/Mn), crystallization temperature (Tc), and melting temperature (Tm), as reported in Table 3 below.

TABLE 3 Properties of Poly. 1 to Poly. 6 Density Melt Tc Tm Polymer (g/cm³) Index (I2) Mw/Mn (° C.) (° C.) Poly. 1 0.950 19.0 2.170 114.50 128.81 Poly 2. 0.950 17.0 3.750 115.00 127.35 Poly 3. 0.968 35.0 3.144 118.23 133.18 Poly. 4 0.935 17.0 2.300 113.20 127.45 Poly. 5 0.935 21.0 2.300 111.80 126.21 Poly. 6 0.907 25.0 2.800 96.32 97.86

Formation of Fibers

Fibers are spun on a Hills Bicomponent Continuous Filament Fiber Spinning Line. Bicomponent fibers having an eccentric core sheath configuration are made. The fibers are spun on the Hills Line according to the following conditions. Extruder profiles are adjusted to achieve a melt temperature of 240° C. Throughput rate of each hole is 0.5 ghm (grams per hour per minute). A Hills Bicomponent die is used and operated at a 40/60 core/sheath ratio (in weight) with the first region comprising an example in one extruder and second region comprising another example in the other extruder, in accordance with Table 4 below, to form Inventive Examples 1, 2, 3, and 4, and Comparative Examples 1, 2, 3, 4, 5 and 6. The Hills Line pressure is set at 40 psi. The die consists of 144 holes, with a hole diameter of 0.6 mm and a length/diameter (L/D) of 4/1. Quench air temperature and flow rate are set at 15-18° C., and 520 cfm (cubic fit per minute), respectively. After the quenching zone, a draw tension is applied on the 144 filaments by pneumatically entraining the filaments in a slot unit with an air stream. Velocity of the air stream is controlled by the slot aspirator pressure.

TABLE 4 Fiber Examples First Second Weight Ratio- Region Region First Region (Core) to Example (Core) (Sheath) Second Region (Sheath) Inventive Ex. 1 Poly. 1 Poly. 5 40:60 Inventive Ex. 2 Poly. 1 Poly. 6 40:60 Inventive Ex. 3 Poly. 3 Poly. 5 40:60 Inventive Ex. 4 Poly. 3 Poly. 4 40:60 Comparative Ex. 1 Poly. 1 Poly. 1 40:60 Comparative Ex. 2 Poly. 1 Poly. 4 40:60 Comparative Ex. 3 Poly. 2 Poly. 5 40:60 Comparative Ex. 4 Poly. 2 Poly. 4 40:60 Comparative Ex. 5 Poly. 3 Poly. 2 40:60 Comparative Ex. 6 Poly. 3 Poly. 3 40:60

Table 5 provides the difference in melting temperature (ΔTm), difference in crystallization temperature (ΔTc), and difference in density (ΔDensity) between the first region minus the second region for Inventive Examples 1 and 2 and Comparative Examples 1 and 2, examples which have a polyethylene composition in the first region with a molecular weight distribution (Mw/Mn) of less than 3.

TABLE 5 ΔTm, ΔDensity, ΔTc Data Example ΔTm (° C.) ΔTc (° C.) ΔDensity (g/cm³) Inventive Ex. 1 2.60 2.70 0.015 Inventive Ex. 2 30.95 18.18 0.043 Comparative Ex. 1 0 0 0 Comparative Ex. 2 1.36 1.30 0.015

Table 6 provides the difference in melting temperature (ΔTm), difference in crystallization temperature (ΔTc), and difference in density (ΔDensity) between the first region minus the second region for Inventive Examples 3 and 4 and Comparative Examples 3, 4, and 5, examples which have a polyethylene composition in the first region with a molecular weight distribution (Mw/Mn) of greater than 3.

TABLE 6 ΔTm, ΔDensity, and ΔTc Data Example ΔTm ΔTc ΔDensity Inventive Ex. 3 6.97 6.43 0.033 Inventive Ex. 4 5.73 5.03 0.033 Comparative Ex. 3 1.14 3.20 0.015 Comparative Ex. 4 −0.10 1.80 0.015 Comparative Ex. 5 5.83 3.23 0.018 Comparative Ex. 6 0 0 0

Certain Inventive and Comparative Examples are tested for % crystallinity in accordance with the Raman microscopy test method described above. Table 7 shows the results.

TABLE 7 % Crystallinity from Raman Microscopy Raman Measured % Crystallinity First Region Second Region Example (Core) (Sheath) Δ Crystallinity Inventive Ex. 3 63.71% 51.97% 11.74% Inventive Ex. 4 64.04% 53.18% 10.86% Comparative Ex. 5 65.51% 60.68% 4.83% Comparative Ex. 6 65.51% 64.38% 1.13%

Table 8 shows the amount of curvature related to the Examples. Inventive Example 1 to 4 have significantly higher curvature than the Comparative Examples, which have no curvature.

TABLE 8 Curvature Data Curvature Example (mm⁻¹) Inventive Ex. 1 1.80 Inventive Ex. 2 2.20 Inventive Ex. 3 1.95 Inventive Ex. 4 0.89 Comparative Ex. 1 0 Comparative Ex. 2 0 Comparative Ex. 3 0 Comparative Ex. 4 0 Comparative Ex. 5 0 Comparative Ex. 6 0

Table 9 provides the centroid off-set and radius of fiber data for certain examples.

TABLE 9 Radius of Fiber and Centroid Off-Set Radius of fiber Centroid Off- Example (μm) Set (P_(r2)/r) Inventive Ex. 3 11 0.30 Inventive Ex. 4 10 0.38 

1. A bicomponent fiber comprising: a fiber centroid; a first region having a first centroid and a second region having a second centroid; the first region comprising a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of less than 3.0; the second region comprising a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid; and wherein the first polyethylene composition has a crystallization temperature (Tc) at least 2° C. greater than a crystallization temperature (Tc) of the second polyethylene composition.
 2. The bicomponent fiber of claim 1, wherein the first polyethylene composition has a melting temperature (Tm) at least 2° C. greater than a melting temperature (Tm) of the second polyethylene composition.
 3. The bicomponent fiber of claim 1, wherein the melting temperature (Tm) of the first polyethylene composition is less than 130° C.
 4. A bicomponent fiber comprising: a fiber centroid; a first region having a first centroid and a second region having a second centroid; the first region comprising a first polyethylene composition having a molecular weight distribution, expressed as the ratio of the weight average molecular weight to number average molecular weight (M_(w(GPC))/M_(n(GPC))), of greater than 3.0; the second region comprising a second polyethylene composition having a density less than a density of the first polyethylene composition; wherein at least one of the first centroid and the second centroid is not the same as the fiber centroid; wherein the first polyethylene composition has a crystallization temperature (Tc) at least 3.5° C. greater than a crystallization temperature (Tc) of the second polyethylene composition.
 5. The bicomponent fiber of claim 4, wherein the first polyethylene composition has a melting temperature (Tm) at least 5° C. greater than a melting temperature (Tm) of the second polyethylene composition.
 6. The bicomponent fiber of claim 1, wherein the density of the first polyethylene composition is at least 0.015 g/cm³ greater than the density of the second polyethylene composition.
 7. The bicomponent fiber of claim 1, wherein the fiber has a curvature of greater than 0.5 mm⁻¹.
 8. The bicomponent fiber of claim 1, where the first polyethylene composition has a Raman measured % crystallinity at least 5.00% greater than a Raman measured % crystallinity of the second polyethylene composition.
 9. A spunbond nonwoven comprising the bicomponent fiber of claim
 1. 